The technical field of the invention is the one of pressurized water nuclear plants. More particularly, the technical field of the invention is the one of steam generators for pressurized water nuclear reactors.
The present invention relates to an anti-vibration (vibration-inhibiting) bar for a steam generator tube bundle.
The sector of pressurized water nuclear reactors uses low-enriched uranium as a fuel and light water as a moderator and a coolant. These reactors are known as “indirect cycle” reactors because they include two distinct circuits: the primary circuit extracts the energy produced by the reactor and transfers it to the secondary circuit, which transforms it into steam and then into electricity.
FIG. 1 shows a steam generator 1 of a pressurized water nuclear reactor. The steam generator 10 includes an external shell 11 in which is provided, inside a bundle wrapper 12, a bundle 13 of tubes. The bundle of tubes is comprised of U-folded tubes 14. Each U-folded tube 14 has a first straight leg 14-1 and a second straight leg 14-2 connected by a semi-circular bend 14-3. The tubes 14 of the bundle 13 are arranged into several juxtaposed banks to constitute the bundle 13. A bank of tubes includes tubes 14 the bends 14-3 of which have radii different from one another and are placed adjacent in a same vertical plane, parallel to the first section plane P1 of FIG. 1. More precisely, the bends 14-3 of a same bank of tubes 14 have decreasing radii from the periphery of the bundle 13 to its centre part.
Thus, the bundle 13 has:                a lower part of a substantially cylindrical shape and including the straight legs 14-1, 14-2 of the tubes 14 of the bundle 13;        an upper part of a substantially hemispheric shape referred to as a tube bend region and including the bends 14-3 of the tubes 14 of the bundle 13.        
FIG. 1 shows an inlet 15-1 and an outlet 15-2 of a fluid circulation primary circuit, as well as an inlet 16-1 and an outlet 16-2 of a fluid circulation secondary circuit. While the steam generator 10 is operating:                high temperature pressurized water arrives through the inlet 15-1 of the primary circuit, circulates inside the tubes 14 of the bundle 13 and comes out through the outlet 15-2 of the primary circuit;        feed water arrives through the inlet 16-1 of the secondary circuit, is brought into contact with the outer surface of the tubes 14, along which it moves in a vertical direction by being gradually brought to the boil, to finally come out at the outlet 16-2 of the secondary circuit as steam.        
The primary fluid circulation inside the tubes 14 and the secondary fluid circulation in contact with the tubes 14 cause the tubes 14 to vibrate. In order to hold the tubes 14 and to prevent their colliding, straight legs 14-1, 14-2 of the tubes 14 are engaged into spacers 17 located at regular distances from one another along the height of the steam generator 10, and the bends 14-3 of the tubes 14 of the bundle 13, which constitute the tube bend region, are held by means of anti-vibration bars 18 each interposed between two banks of adjacent tubes of the bundle 13 and disposed along a radial direction of the tube bend region.
These anti-vibration bars 18 are generally hingedly connected in twos at their end 18-1 disposed inward of the tube bend region to constitute V-shaped structures. The outer ends 18-2 of the anti-vibration bars opposite their hinged end 18-1 are generally protruding with respect to the tubes constituting the external layer of the tube bend region. These outer ends 18-2 are generally fixed on fixing elements 19 placed on the upper surface of the tube bend region. The anti-vibration bars 18 therefore hold the bends 14-3 of the tube bend region for a maximum limitation of their vibrations, while enabling their expansion when the steam generator 10 is operating.
In order to facilitate the assembly of the tubes in the steam generator 10, an assembly clearance between the anti-vibration bars 18 and the bends 14-3 of the bundle is necessary. This clearance causes, when the steam generator 10 is in operation, residual vibrations of the bundle bends. These residual vibrations are accompanied by brief intense impacts and frictions generating premature wear and deterioration of the bundle bends at the contact areas with the anti-vibration bars.